Electrolyte for the electrodeposition of palladium



United States Patent Ofiice 3,3762% Patented Apr. 2, 1968 3,376,206 ELECTRGLYTE FOR THE ELECTRODEPOSITION F PALLADIUM George J. Kahan, Port Washington, and David W. Hall,

Wappingers Falls, N .Y., assignors to International Business Machines Corporation, New York, N.Y., a corporation of New York No Drawing. Filed June 30, 1964, Ser. No. 379,395 2 Claims. (Cl. 204-47) ABSTRACT OF THE DISCLOSURE Palladium coating processes and baths using water soluble salts of palladium with strong acids, buifering salts of weak bases and strong acids and organic complexing agents at a pH between 4 and 5.5, e.g., PdCl NH CI and disodium ethylenediamine tetraacetic acid, for preparing highly catalytically active palladium films for subsequent electroless deposition of magnetic films are described herein.

This invention relates to the electrodeposition of catalytically active palladium on substrates and more particularly, to the electrodeposition of palladium onto a closely related class of metals and their alloys including copper, nickel and silver and especially to the electrodeposition of palladium on copper.

The prior art is aware of the fact that difficult and, often unique, problems are involved in the deposition of palladium upon substrates by electrolytic deposition. These problems may relate to the structure of the palladium deposited and the physical and chemical nature of the deposit. For instance, it has been observed that the deposition of palladium by the immersion dip coating of palladium onto copper surfaces from plating solutions of palladium salts results in deposits containing nodules of dendritic crystalline structure. When nickel-iron alloys are deposited by an electroless process over such platings, it has been observed that irregular or rough surfaces result which may sometimes extend to distances of thousandths of an inch from the surface of the copper. As a result, such palladium deposits on copper are quite unsatisfactory for catalytically activating the deposition of smooth magnetic films so important in the production of chain store devices.

In accordance wtih the preponderant teaching of the prior art, the electrodeposition of palladium advantageously takes place from electrolyte solutions of palladium salts either under neutral or slightly alkaline conditions or, in the alternative, at relatively low pHs of 2 and below where heavy ductile coatings are desired. It has been taught, however, that such acid baths are not suitable for direct plating of palladium onto nickel, copper, iron, silver and the like. In these prior art platings, the platings were either catalytically inactive for electroless deposition or undesirable cementation reactions took place.

It is a special object of this invention to provide electrolytic baths and a process for the electrodeposition of catalytically active palladium platings which activate the electroless deposition of very smooth and even deposits of magnetic films.

It is another object or" this invention to provide a bath and process for the electrodeposition of thin, or heavy, deposits of palladium which are highly reflective.

It is a more particular objective of this invention to provide electrolytic baths and a related process for the electrodepositing of smooth deposits of palladium on copper which catalytically activate the electroless deposition of a corresponding smooth magnetic NiFe film.

It in another special object of this invention to provide an electrolytic bath suitable for the electrodeposition of palladium under mildly acid conditions at pHs between about 4.0 to 5.5 and especially at pHs of about 5.

Other objects, features and advantages of the invention will become apparent from the description which follows.

It has been found, surprisingly, that the objects of this invention can be obtained by the electrodeposition of palladium from aqueous solutions of water soluble salts of palladium at pHs between about 4.0 to 5.5. In the preferred practice of the invention water soluble salts of palladium with strong acids such as the inorganic mineral acids or the organic low molecular fatty acids are used in the presence of a quantity of an organic complexing compound, or agent, suflicient to form a complex with the palladium salt used. For practical reasons, palladium chloride customarily is used but other water soluble salts such as palladium sulphate, nitrate, acetate, or the like may be used. For the same reason disodium ethylenediamine tetraacetic acid (EDTA) customarily is used but other known complexing, or sequestering, agents may be used.

The concentration of the palladium salts in the plating baths of this invention may be varied widely in conformity with known practice and in keeping with the solubilities of the salts. The higher concentrations favor greater permissible current densities but the concentrations usually are selected so as to make the most effective utilization of palladium present in the bath. Operable concentrations of the palladium salts customarily fall between about .01 to .121 mols, per liter. The amount of complexing or sequestering agent is correlated with the amount of palladium salt and accordingly normally would be about 2 mols of the sequestering, or complexing, agent per mol of the palladium salt.

As is the practice with other electrolytic solutions operating at a desired pH, buffering agents are used. Since the electrodeposition is to take place under mildly acid conditions, the buffering agent customarily used is the salt of a strong acid with a weak base typified by ammonium chloride. Other suitable buffering salts include phosphate buffers such as mixed alkali-acid phosphates. Of course the presence of depositable metal ions other than palladium should be avoided.

Where the desired pH is not readily obtainable by the use of buffering salts alone, strong acids or weak bases may be used to adjust the pH to the desired value. For example, where ammonium chloride is used as the buffering salt, ammonium hydroxide or hydrochloric acid may be used to adjust the pH to a desired value without introducing extraneous ions which may affect the electr0deposition process or the controls associated therewith.

In the electrodeposition process of this invention, temperature controls have been found not to be important since electrodeposition takes place very satisfactorily at room temperatures without temperatures rising deleteriously. Where more rapid deposition is desired, the temperature of the electrolytic bath may be mildly raised. It is obvious temperatures above the boiling point of the plating bath should be avoided.

Current densities conventionally used in the electrodeposition of palladium are quite satisfactory. For instance, current densities between about 3.6 amps/ft. and 18 amps/ft. at 23 C. are quite satisfactory or between 5.8 amps/ft. and 46 amps/ft. at C.

In the operation of the process of this invention, palladium may be supplied by be advantageously agitated in a conventional manner, as by bubbling air through the bath.

Since palladium chloride, for economic and operational reasons, is a preferred electrolyte suitable baths using it are set out below in tabular form.

the anode and the bath may Constituent Low, m./l. Preferred, m./1. High, m./1.

PdClz .01 .0705 .121 NH Cl .04 .234 Saturated EDTA .02 .141 .242 HCl and NH OH are used, where necessary, to adjust to pH of 5.0.

Since the present invention is importantly concerned with the electrodeposition of palladium on copper, a specific example will be given of such a process for illustrative purposes. It will be understood, however, the same bath and the same operable conditions could be used to electrodeposit palladium on nickel or silver or alloys of copper, such as beryllium copper or cadmium copper or alloys of nickel or alloys of silver to obtain a very smooth, uniform catalytically active and highly reflective deposit.

It will be further understood that uniform electrodeposited platings can be obtained from the baths of this invention upon suitable cathodes conforming to the surface characteristics thereof.

It will be understood the following example merely is exemplary.

Example An aqueous solution was prepared with ingredients listed below:

g./l. PdCl 12.5 NH Cl 12.5 EDTA 110.0

HCl or NH OH as needed While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. An aqueous bath suitable for the electrodeposition of uniform and highly reflective catalytically active coatings of palladium consisting essentially of the following ingredients per liter about .01 m. to 121 rn. of PdCl about .04 m. to a saturated amount of NH Cl and about .02 m. to .242 m. of a chelate selected from the group consisting of ethylenediamine tetraacetic acid and alkali metal salts thereof, said bath having a pH of about 4 to 5.5.

2. An aqueous bath suitable for the electrodeposition of uniform and highly reflective catalytically active coating of palladium consisting essentially of the following ingredients per liter 12.5 g. of PdCI 12.5 g. of NH CI and g. of disodium ethylenediamine tetraacetic acid, said bath having a pH between about 4 and 5.5.

Atkinson, R. H., et al. The Electrodeposition of Palladium, Electrometallurgy (Supplement to the Metal Industry), pp. 595-598, June .9, 1933.

HOWARD S. WILLIAMS, Primary Examiner.

JOHN H. MACK, Examiner.

G. KAPLAN, Assistant Examiner.

Metal Finishing Guidebook Directory, pp. 382-384, 

